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Experiments to try at home
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out about forces and motion and Newton's three laws in the comfort of
your own home. |
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What are
forces?
Measuring forces
Friction
Experiment with inertia
Experiment with forces and acceleration
Experiment
with balloon rockets
What are forces?
Forces are pushes
and pulls and are happening all the time. You cannot see forces but you
can see their effects, sometimes they are very dramatic. Think about all
the forces you experience everyday: when you pull open drawers, when your
bag full of shopping pulls on your hands, when you give a football a push
to score a goal, even the sails of a windmill are being pushed around by
the wind. Can you think of any other pushes and pulls that you feel every
day?
Measuring forces
You don't need any special
equipment to measure forces - just some elastic bands and a ruler. Forces
are measured in Newtons after Sir Isaac Newton, but here you will see the
effects of forces in centimetres.
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To make a force measurer, you will need:
1 empty margarine tub or other small box
3 elastic bands of the same size
String
30 marbles
1 ruler
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- Tie a piece of string tightly around the box.
Attach one of the elastic bands to this string. Now attach the
other two elastic bands to the first one in a chain.
- Put the box on a level surface such as the
carpet. Straighten out the chain of elastic bands, but do not
stretch them yet. Put the ruler so that '0 mm' lines up with the
end of the elastic band chain.
- Put 15 marbles into the box. Slowly pull the
end of the elastic band chain. Make a note of the distance the
elastic bands have stretched at the moment the box begins to move.
- Put the other 15 marbles
into the box and pull the end of the elastic band chain again.
Make a note of the stretch on the elastic bands when the box
begins to move this time.
Did the elastic bands stretch more or less with more
marbles in the box? To pull more marbles you must use a bigger force, so
the elastic bands stretched more. Try measuring the force needed to make
other things move. Which do you think will need more force - a stack of
books, a full bottle of water, a skateboard?
Friction
You can use your elastic bands and ruler to investigate
friction on your shoes. Friction is a force that slows things down. It is
the force that stops your shoes from slipping when you walk.
To measure friction in your shoes, you
will need:
A selection of shoes eg. a wellington boot, a
trainer, a slipper, a walking shoe
Your elastic band force measurer and ruler
Marbles
Weighing scales
String |
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- Look at the soles of the shoes and try to predict which shoe will be
the most grippy.
- To make the test fair all the shoes must weigh the same. Weigh each
of the shoes to find out which shoe is the heaviest. Make a note of how
much this shoe weighs. To make each shoe weigh the same put marbles into
the shoes as required.
- Thread a piece of string through one end of your elastic band chain.
Tie this string round one of the shoes. Straighten out the chain and put
the '0 mm' of your ruler at the end like you did before.
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Draw a table to record your results in. It should look
something like this:
| Shoe |
Extension (cm) |
| Trainer |
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| Wellington boot |
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| Sandal |
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| Walking shoe |
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| Slipper |
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- Pull on the elastic band chain and make a note in your table of how
much the elastic bands have stretched at the moment when the shoe starts
to move. Repeat with all of the shoes remembering to make each shoe
weigh the same.
- Sort out the shoes in order of grippiness. Remember that the shoes
with the most friction are the ones with the most stretch on the elastic
band chain. Were your predictions correct?
Do the shoes stay in the same order of grippiness on a different floor
surface? Friction always depends on two surfaces, in this case the sole of
the shoe and the carpet. You may have found out that it is not always the
rough surfaces that have the most friction. Friction mostly depends on the
two surfaces 'sticking' together. So smooth surfaces can be very grippy -
feel how grippy a pencil rubber is. Do any of your shoes have smooth,
grippy soles?
Newton's first
law and inertia
Newton was one of the first people to think that moving
objects do not need a constant force pushing or pulling them. This was a
radical idea as most people thought that a moving object had 'impetus'.
People thought that when this impetus ran out the object stopped moving.
Newton's idea was that a moving object would keep on moving if a force did
not change its speed or direction. By the same thinking, he suggested that
a stationary object would not start moving unless a force acted on it. You
could think of inertia as a kind of 'inherent sluggishness'.
You can see the effects of inertia in the experiment below.
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To see inertia in action you will
need:
1 Mug
Water
Food colouring
Vegetable oil
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- Half-fill the mug with water. Carefully pour some vegetable oil into
the mug so that it forms a thin layer on the surface of the water. Place
a couple of drops of food colouring onto the oil.
- Hold the rim of the mug from above. With a quick twist of your wrist
turn the mug a quarter turn. What happens to the drops of food
colouring?
You should see that the mug moves, but the drops of food colouring stay
in the same place. This is inertia in action. The mug starts to move
because you exerted a force on it. But you did not exert much of a force
on the liquids in the mug because the friction between the liquids and the
mug is low. The mug slides past the liquids which stay where they are. The
inertia of the liquids makes them stay put.
See Can you explain this?
for more inertia experiments.
Newton's second law - forces and acceleration
Newton's second law states that if you exert a constant
force on an object it will speed up or slow down (accelerate). You can see
this for yourself with the help of a friend and a bike.
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You will
need:
A friend
A bike or skateboard
Open space
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- Find a large open space, but be careful of traffic and other people.
- Get your friend to sit on the bike or skateboard.
- Push your friend along carefully. Try to do this with a constant
force. Your friend must not pedal or use the brakes. If you have got a
spring that is strong enough you can attach one end of the spring to the
bike while you pull them along with the other end. Make sure that the
extension on the spring stays constant so you know you are pulling with
a constant force.
- Keep pushing (or pulling) and see what happens to your speed. You
should find that if you started off walking you have to walk quicker and
quicker, and eventually run, to keep your force constant. This is a
demonstration of constant force resulting in acceleration.
What happens to your friend's speed when you stop pushing? Does he or
she speed up, slow down, or stay the same? You should find that your
friend slows down, so there must be force acting on him or her.
It's friction again. Can you work out where the friction is acting?
There is friction from the air, in the bearings of the axles and the
wheels against the floor.
Newton's
third law - balloon rockets
You may remember something about 'equal and opposite forces' from school?
Well, it is a bit of Newton's third law. Rockets use Newton's third law to
make them lift off the ground and escape earth's gravity. Newton's third
law states that 'for every action, there is an equal and opposite
reaction.' The action is the jet of gases escaping from the rocket
engines. The reaction is the movement of the rocket upwards in the
opposite direction to the gas.
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To make your own rocket at home and see Newton's
third law in action you will
need:
Balloons of different shapes
Long piece of smooth string (5m) to act as a track for
your rocket
Drinking straw cut into 5cm lengths
Sticky tape
Scissors
2 chairs or 2 friends
Thin card
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- Thread the string through the straw. Attach the string to the chairs
making sure it is taught and level. Or you can ask your friends to hold
the string at either end and pull it taught.
- Blow up a balloon and hold it closed so the air does not escape - do
not tie it shut. Attach the balloon to the straw with sticky tape.
- Move your balloon rocket to one end of the string and let go. How
far does your rocket travel?
- Try the same experiment with different shaped balloons. Which shaped
balloon makes the best rocket?
- You can decorate your rocket using the card to make fins and a nose
cone. Does this make your rocket any better?
The balloon rocket works in the same way as an ordinary rocket. The
only difference is that the balloon is travelling along the string and not
going up into the sky.
You can feel the effects of Newton's third law by putting your skates
on. You and a friend should put on some roller skates. Stand facing each
other and put your hands together. Push gently against each other and you
will see that you both move backwards. If you are the same weight you will
move the same distance backwards. If you are heavier than your friend,
then your friend will move back further than you. | 
